The ellipsoidal shells are widely used in thin-walled containers such as large water towers, petrochemical containers, and cylindrical container heads due to the advantages of the small wind area, large capacity, uniform stress and attractive outline, etc. With the increasing requirements of lightweight, large-diameter aluminum alloy ellipsoidal shells with high specific strength and good corrosion resistance are gradually widely used, such as the aluminum alloy ellipsoidal dome of launch vehicle fuel tanks, the aluminum alloy ellipsoidal dome of upper-stage fuel tanks, and marine liquefied gas aluminum alloy storage tanks, etc.
The conventional forming method of an ellipsoidal shell is generally performed by first petal forming in the dies, and then assembling and welding the petals into an integral ellipsoidal shell. Since the curvature radius of the ellipsoidal shell gradually changes from the polar point to the equator, an ellipsoidal shell with one size diameter also requires multiple sets of dies when the conventional die forming is used, which limits the wide application of the ellipsoidal shell. To solve the manufacturing problems existing in the conventional method for the ellipsoidal shell, an overall dieless hydroforming method for the ellipsoidal shell has been developed. The method includes first cutting a plate into several side petals and two polar plates, roll-bending the side petals into single-curvature shell plates of a certain shape, and then assembling and welding the single-curvature shell plates and the polar plates into a closed shell inscribed with an ellipsoid, and finally filling a liquid medium to pressurize the polyhedral shell into an ellipsoidal shell. The method does not require a die and a press, and thus can significantly reduce the manufacturing costs of the large-diameter ellipsoids, and can realize the dieless hydroforming of ellipsoidal shells with different axial ratios. At present, dieless hydroforming has successfully produced low carbon steel and stainless steel ellipsoid, which is applied to water supply, paper making, liquefied petroleum gas, architectural decoration, urban scenic spots, and flight simulation ball screens in the field of national defenses, etc.
However, due to the low room temperature formability of the aluminum alloy, especially the strength and formability of the welded joint are much lower than those of the base metal (the strength of the aluminum alloy fusion-welded joint is 60% of the base metal, and the friction stir welded joint is 80% of the base metal). It is easy to cause the rapture defects in the weld zone when the dieless hydroforming is performed at room temperature, resulting in failure to form a qualified ellipsoidal shell. To improve the formability of aluminum alloy and its welded joint, it is necessary to carry out the forming at elevated temperature. For large-diameter ellipsoidal shells (e.g., the diameters greater than 3 m, even up to 10 m), if heating is carried out in a heating furnace, a large-sized heating furnace is required. Thus the equipment cost is extremely high, and moreover, the temperature uniformity of the large-sized heating furnace is strictly controlled That is, the technical risk is very high, and it is difficult to execute the gas pressure forming in the large-sized heating furnace.